Definition of Hormone

Hormone: A chemical substance produced in the body that controls and regulates the activity of certain cells or organs.

Many hormones are secreted by specialized glands such as the thyroid gland. Hormones are essential for every activity of daily living, including the processes of digestion, metabolism, growth, reproduction, and mood control. Many hormones, such as the neurotransmitters, are active in more than one physical process.

Examples of hormones include aldosterone, antidiuretic hormone(ADH), cortisol, erythropoietin, estrogen, human chorionic gonadotropin (hCG), parathormone, progesterone, and testosterone.

A hormone originally denoted a chemical made by a gland for export to another part of the body. Now a hormone is more broadly any chemical, irrespective of whether it is produced by a special gland or not, for export or domestic use, that "controls and regulates the activity of certain cells or organs.". The word "hormao" which means "I set in motion" or "I stir up" was used in ancient Greece to covey the "vital principle" of "getting the juices flowing." The word "hormone" was resurrected in 1902 (not 1906, as the Oxford English Dictionary states) by the English physiologists Wm. M. Bayliss and Ernest H. Starling who that year reported their discovery of a substance made by glands in the small intestine that stimulated pancreatic secretion. They called the substance "secretin" and dubbed it a "hormone", the first known hormone.